Aren't I?

In all but the most formal situations.  Here is some online commentary:

The technically ungrammatical usage of "aren't" in questions where "I" is the subject can be considered a "forced" mistake, because there is no good grammatically correct alternative. There is no contraction for "am not;" we don't say "Amn't I going ... ?," and the uncontracted form, "Am I not going with you?," sounds so formal that it is not practical in most situations. Consequently, even though "aren't I" is technically ungrammatical, and can sound a bit jarring to the ear, it is considered correct in normal conversational speech.

Additional note:
Michael Swan: "Practical English Usage" Second Edition
"'Am not' is normally only contracted in questions, to 'aren't'"
"The question tag for 'I am' is 'aren't I'?: 'I'm late, aren't I?'"

Still substandard-- and gauche besides.  An excerpt from the Am Heritage Dict:

'...riticism of ain't by usage commentators and teachers has not subsided, and the use of ain't is often regarded as a sign of ignorance. •But despite all the attempts to ban it, ain't continues to enjoy extensive use in speech. Even educated and upper-class speakers see no substitute in folksy expressions such as Say it ain't so and You ain't seen nothin' yet. •The stigmatization of ain't leaves us with no happy alternative for use in first-person questions. The widely used aren't I? though illogical, was found acceptable for use in speech by a majority of the Usage Panel in an earlier survey, but in writing there is no acceptable substitute for the stilted am I not?'